The National Research Council's Workshop on adaptation to climate change impact on Urban / rural storm flooding February 27, 2018. Presentation on: National, regional, local IDF trend analysis & hyetograph selection to define risks and system stresses Robert J. Muir, M.A.Sc., P.Eng. Manager, Stormwater, City of Markham

1. Woodbine
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Workshop on adaptation to climate change impact on
Urban / rural storm flooding
February 27, 2018
National, regional, local IDF trend
analysis & hyetograph selection to
define risks and system stresses
Robert J. Muir, M.A.Sc., P.Eng.
Manager, Stormwater, City of Markham

2. Woodbine
Outline
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• Environment and Climate Change Canada (ECCC) Engineering
Climate Datasets (version 2.3)
– Annual Maximum Series Trends and Significance
– Regional Trends In Observed Extremes
• Local IDF Trends Since 1990
• Future IDF Uncertainty
• Hyetograph Selection as Critical Risk Factor (how IDF data is
applied in practice)

3. National Annual Maximum Rainfall Trends Data
• ECCC added trend analysis in
the v2.3 Engineering Climate
Dataset, showing trend
direction and statistical
significance at each station:
http://climate.weather.gc.ca/prods_servs
/engineering_e.html
• Ottawa Airport
 Decrease 5-min to 6-hrs.
 Increase 12 to 24 hrs.
 Statistically significant
decrease over 10-min,
15-min, and 1-hr.
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4. Very Few Significant Trends in Canada (random)
• 93% of trends are not
significant or ‘no data’.
• 5-min rainfall maxima
have significant increases
at 2.7% of stations.
• Decreasing regional
trends in southern QC
and Atlantic. Increasing
on coasts (SW BC and
NFLD).
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Significantly
Down 3%
Significantly
Up 4%2.7%
http://www.cityfloodmap.com/2015/12/trends-in-canadian-shortduration.html

5. 5http://www.cityfloodmap.com/2015/12/canadian-extreme-rainfall-map-climate.htmlhttp://www.cityfloodmap.com/2015/12/canadian-extreme-rainfall-summary-by.html

6. National Data Trends Contradict Media & Marketing
• Insurance ‘facts’ & infographics
refuted by ECCC as inadequate
for decision making on risks:
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• Data facts from ECCC:
– “No Detectable Trend Signal” (Atmosphere-
Ocean, 2014)
– “No significant change in rainfall events over
several decades” (CBC letter, 2015)
– Advertising Standards Canada complaint
resolutions 2015-2016
– “ECCC studies have not shown evidence to
support statement” (Cdn Underwriter, 2016)
– “If this is used as the basis for statements
about actual changes in extreme rainfall in
Canada, then I would have concerns.”
(personal communication ECCC, 2018)
http://https://www.slideshare.net/RobertMuir3/storm-intensity-not-increasing-factual-review-of-engineering-datasets

7. Mixing-up Annual Precip. Data and Extreme Rainfall Risks
“Indeed in Canada, in southern Canada we are getting about 18% more
rainfall on an annual basis than was the case just over 100 years ago. So
when you see in the news and the media people talk about storms seem
bigger and more intense and so forth, those perceptions are correct. And
there's a lot of data to show it. I'm just giving one quick illustration here.”
Blair Feltmate, ICCA, February, 8, 2018
Standing Senate Committee on Energy, the Environment and Natural Resources
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8. Less Extreme Short Duration Rain - Southern Ontario
• 97 % of trends not statistically significant (mild trends up & down).
• 2.3% statistically significant decreases and only 1.0 % significant
increases in intensity. No short duration significant increases.
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97%
http://www.cityfloodmap.com/2016/02/ontario-climate-change-trends-going.html

9. No Change in Annual Maxima = No Change in Derived IDF
• Southern Ontario 21
stations with 30+ years
of record have median
IDF changes since 1990:
– 2-Yr 5-Min -0.5 %
– 5-Yr 10-Min 0.0 %
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http://www.cityfloodmap.com/2018/01/short-duration-frequent-rainfall-show.html

10. No Change in Annual Maxima = No Change in Derived IDF
• Southern Ontario 21
stations with 30+ years
of record have median
IDF changes since 1990:
– 2-Yr 5-Min -0.5 %
– 5-Yr 10-Min 0.0 %
– 5-Yr 1-Hr -1.2 %
– 10-Yr 2-Hr -0.6 %
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http://www.cityfloodmap.com/2018/01/short-duration-frequent-rainfall-show.html

11. Local Trend - Lower 5-Minute IDF For 2 to 100 Year
• Toronto Downtown
and Mississauga
Airport 5-15 min.
intensities decreasing
for all return periods
(like 5 minute data in
this table).
• 24 hour trends are
downward as well
despite July 8, 2013
storm.
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12. Southern Ontario Extreme Rain Trends Decreasing
http://www.cityfloodmap.com/2017/11/less-extreme-short-duration-rainfall-in.html
http://www.cityfloodmap.com/2017/09/less-extreme-ontario-rainfall.html
http://guelph.ca/wp-content/uploads/SMMP-Appendix_E_Combined_IDF_Report.pdf
“new rainfall depths are lower
than current by 10 to 15 %”,
EarthTech
https://drive.google.com/open?id=1Gxmg8gtkzZuv-ZqiqYpc3tQ9r-Ie1v1p 12https://drive.google.com/open?id=1AngUYFFlm-RqQlmSC0gZqxy8nV61BW8J

13. 100-Year 15-Minute
Future IDF
Uncertainty
• Various options:
– Tool / Method
– Station / Cell
– RCP/ Conf.
Bands
– Timeframe
• Wide diversity in
future volume
across options.
• No consistency in
governing option
across durations.
IDF CC Tool
2050-2100
CCDP
2065-2095
RCPs RCPs RCPs RCPs 10-90% 10-90% 10-90%
Local Climate Stations Local Grid Cells
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14. 100-Year 15-Minute
Future IDF
Uncertainty
• Various options:
– Tool / Method
– Station / Cell
– RCP/ Conf.
Bands
– Timeframe
• Wide diversity in
future volume
across options.
• No consistency in
governing option
across durations. 15
H
L

15. 100-Year 2-Hour
Future IDF
Uncertainty
• Various options:
– Tool / Method
– Station / Cell
– RCP/ Conf.
Bands
– Timeframe
• Wide diversity in
future volume
across options.
• No consistency in
governing option
across durations. 16
H
L

16. 100-Year 24-Hour
Future IDF
Uncertainty
• Various options:
– Tool / Method
– Station / Cell
– RCP/ Conf.
Bands
– Timeframe
• Wide diversity in
future volume
across options.
• No consistency in
governing option
across durations. 17
H
L

17. Is IDF data
critical to
design?
Does it
account for
real system
stresses?
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18. Is IDF data
critical to
design?
Does it
account for
real system
stresses?
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19. IDF data is
just the “tail
on the dog”
... don’t let
“the tail wag
the dog”
when
defining risk
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20. IDF Data Use (To Create Hyetograph) is Most Critical to Risk
• Regulatory floodplain hyetographs can underestimate 100-year rain by 90%.
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0
50
100
150
200
250
300
0 100 200 300 400 500
RainfallIntensity(mm/hr)
Time (minutes)
Markham Storm (3 Hr
AES) 100 Year
TRCA Storm (AES 12
Hr) 100 Year
Markham 100-Yr
IDF 100-Yr
TRCA 100-Yr
Markham 100-Yr
TRCA 100-Yr
Risk
Gap

21. Hyetograph Selection Effects Major Runoff Peaks (Storm)
• Various 100-year storms to assess peak flows in Toronto InfoWorks models.
• “Chicago” distribution gives higher overland, major system design flows:
• 108% higher in small catchment
• 60% higher in medium catchment
• 36% higher in large catchment
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Description
Minor Peak (m3/s) Major Peak (m3/s)
Chicago AES Chicago AES
Area 15 – Small Catchment 0.16 0.15 0.25 0.12
Area 15 – Large Catchment 3.74 3.66 0.45 0.33
Area 17 – Small Catchment 0.12 0.10 0.34 0.22
Area 17 – Medium Catchment 2.79 2.72 0.32 0.20
Area 17 – Medium Catchment 1.28 1.25 1.49 1.01
Area 17 – Large Catchment 49.09 48.37 0.07 0.05

22. Hyetograph Selection Effects Surcharge (Sanitary)
• Several wastewater design storms were used to assess surcharged manholes
representing basement flooding risks in Markham’s all-pipe InfoWorks model.
• “Chicago” storm has highest surcharge/flood potential (conservative):
– 20 times more flood risk compared to historically-based design storms.
– 4.3 times more flood risks compared to Region operational / trunk storm.
– Higher risk for Today’s IDF than other storms with Future IDF (stress tests
for Climate Change resiliency, demonstrates safety factors in design)
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Design Storm
# MH Flooded
(< 2m freeboard)
% MH Flooded
(< 2m freeboard)
York Region 25-Year 'Historical' 60 0.42 %
Toronto May 2000 'Historical' (25-50 Year) 51 0.35 %
Ottawa 100-Year 'Historical / Scaled' 13 0.09 %
Markham 100-Year Chicago 259 1.80 %

23. Hyetographs & Hydrology have safety
factors to account for complex reality
• Chicago storm: “the synthetic storm pattern would
undoubtedly have a greater return period than the
IDF curve from which it was derived” (Marsalek
and Watt, 1984)
• “the science of hydrology has evolved into a
practice that relies heavily on empirical relation-
ships or concepts, that describe the major
hydrologic process in a relatively gross way…The
present best state of the art in this field still relies
heavily on the use of judgement of the practitioner
to balance the complex reality with the need for
useful simplifications. “ (Ryerson University)
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24. Conclusions
• Newest ECCC Engineering Climate Datasets Version 2.3 trends and
significance for each station/duration define extreme rain intensity changes:
– Short duration intensities define urban flood risk (not annual precip.)
– Local station and even regional trends may be up or down.
– Local IDF changes follow observed intensity trends (e.g., lower IDF
values in southern per Ontario ECCC datasets & countless studies).
• Media / marketing focus on rainfall trends diverts focus from key flood risk
drivers and the most effective mitigation measures (see next presentation).
• Future IDF predictions high variable (but not critical to define cost effective
mitigation measures due to certainty in existing system limitations)
• Hyetograph selection more critical to defining risk (i.e., design storm to set
level of service for mitigation) than IDF refinements.
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